The three-dimensional structure of a snake venom curaremimetic neurotoxin erabutoxin b was first determined in this laboratory. There are more than fifty such protein postsynaptic neurotoxins in the venoms of both land and sea snakes; they show extensive sequence homologies. From a study of these homologies and the effect of replacement (in deviant toxins) and/or of the chemical modification of certain residues, the reactive site has been defined and the reactive groupings characterized. The model of antagonist activity thus developed here following the determination of the stereochemistry of erabutoxin b, which has both general and acetylcholine mimetic binding groups, is now under study. The implied topological invariance must be investigated. The X-ray crystal structure analyses of both normal and deviant long toxins and other normal and deviant short toxins are planned. High resolution refinement of the erabutoxin b structure is in progress. It will be used initially in Molecular Replacement procedures for the crystal structure analyses of data from crystals erabutoxin c and laticotoxin a now already prepared.